Automotive radio rundown noise eliminator

ABSTRACT

A circuit is incorporated into an electronically tuned radio receiver for eliminating rundown noise. As the power supply voltage in an electronically tuned radio receiver declines, the radio does not merely fade away, it also has its tuning swept thereby producing an undesirable sound, herein termed &#39;&#39;&#39;&#39;rundown noise.&#39;&#39;&#39;&#39; This can occur in electronically tuned solid state radio receivers that employ very large power supply filter capacitors which will operate the radio for a short period of time after the primary power supply has been terminated. In automotive systems the existence of an electric-motor-containing accessory, such as a heater or air conditioner, may produce rundown noise. If the motor is running while the radio is in operation, turning the ignition switch off removes power to the system but the electric motor, while coasting to a stop, acts as a d-c generator thereby supplying a declining voltage to the radio. The circuit of this invention terminates radio operation abruptly when the power supply voltage drops below a predetermined value.

United States Patent 11 1 Harris 14 1 Sept. 25, 1973 Primary Examiner--Robert L. Griffin Assistant Examiner-Marc E. Bookbinder Attorney-Robert D. Sanborn [75] Inventor: Wayne C. Harris, Ambler, Pa.

[73] Assignee: Philco-Ford Corporation, [57] ABSTRACT Philadelphia, A circuit is incorporated into an'electronically tuned [22] Filed July 27 1971 radio receiver for eliminating rundown noise. As the power supply voltage in an electronically tuned radio [21 Appl. No.: 166,516 receiver declines, the radio does not merely fade away,

it also has its tuning swept thereby producing an unde- 52 us. c1 325/313 307/320 325/478 l This 325/493 334/5 can occur in electronically tuned solid state radio re- [51] Int CL 6 1/10 ceivers that employ very large power supply filter ca- [58] Fie'ld 313 348 pac tors WhlCh will operate the radio for a short period 325/362 492 2 of time after the primary power supply has been termi- 318 319 3 1 5 1 5 nated In automotive systems the existence of an 340/2423 A 328/258 2 electric-motor-containing accessory, such as a heater 302/264 6 or air conditioner, may produce rundown noise. If the f motor is running while the radio is in operation, turning 5 6] References Cited the ignition switch off removes power to the system but the electric'motor, while coasting to a stop, acts as a d-c UNlTED STATES PATENTS generator thereby supplying a declining voltage to the 3,440,544 4/1969 Pampel 325/422 radio. The circuit of this invention terminates radio opg l et eration abruptly when the power supply voltage drops atorre 3,189,830 6/1965 Jenkins et a1... 325/313 below a predetermined value. 2,617,019 11/1952 Hepp 325/348 5 a 2 Drawing g s 2,324,424 7/1943 Pollack 340/248 B 3,583,856 6/1971 Bauer et al 340/248 B 2.678391 5/1954 Lappin 328/259 Aura mwr/a/v Ara-1x04) 4 5477!) up; 00f! |l|ll{c EFL} 2 M AAfl/U RICE/V5? l? M 3 A 177/707 m In fll/Vifl r/u (Vial/7f 6 Al/fl/O BACKGROUND OF THE INVENTION In recent years electronically tuned radio receivers have risen in popularity. In view of the advantages of such a tuning system it appears that an increasing percentage of radios will use it. In applying such radios to automotive use, a problem peculiar to automotive power systems has been discovered. The motors used in automotive accessories are low voltage (usually 12 volts), direct-current, high-efficiency devices. Such motors, when driven or coasting, act as dc generators. If a motor-containing accessory is turned on and its motor is running, turning the ignition switch off will remove power to the motor and it will coast to a stop.

It has been discovered that when the ignition switch is turned off, the coasting motor will apply as much as 7 to 8 volts to the power line. This voltage then slowly drops to zero as the motor coasts to a stop. If the radio has been operating, the motor will run it for a few seconds at reduced output. This is unobjectionable in con ventionally tuned radios but creates a special problem for electronically tuned radios, in which tuning is a function of applied voltage. Such a radio will not simply die away but, due to the declining voltage, will tune across the band as it dies out and create a rather unpleasant cacophony of sound, herein termed rundown noise.

A similar condition exists in solid state receivers in general where electronic tuning is employed. Such receivers commonly employ very large filter capacitors which store a substantial quantity of energy. When such a receiver is turned off the capacitors can supply sufficient energy to maintain radio operation for up to several seconds. As the capacitors discharge, the operating voltage declines and the electronic tuning is swept, thereby giving rise to rundown noise.

The rundown noise problem is particularly aggravated in radio receivers employing circuits designed to have voltage tracking. Such circuits are arranged so that bias voltages follow changes in Supply voltages to provide for correct operation over a wide range of'supply voltage. Such circuits will operate at supply voltages of quite low value, and, while this is desirable from a receiver operation standpoint, the rundown interval is extended and the associated noise more evident.

SUMMARY OF THE INVENTION It is an object of this invention to eliminate, in an electronically tuned automobile radio, the rundown noise associated with motor-containing accessories.

It is also an object of this invention to eliminate, in electronically tuned solid state receivers, the rundown noise associated with large power supply filter capacitors.

It is a further object to eliminate rundown noise by means of a switching circuit that responds to the reduced supply voltage that occurs during rundown.

These and other objects are achieved with an automatic switch that is designed to sense the reduction in voltage that accompanies rundown and to switch the radio offimmediately. This can be achieved by a simple electrical relay or its electronic latch equivalent. Alternatively the switching actioncan be incorporated into circuits already present in the radio.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, FIG. 1 is a partial schematic block diagram of an automobile radio employing voltage variable capacitors tuning, and the power supply system, showing one embodiment of the invention; and

FIG. 2 shows a second embodiment of the invention.

DESCRIPTION OF THE INVENTION FIG. 1 shows the automobile elements of interest in relation to a prior art radio switching system, and one means for practicing the invention is shown connected between points A and B. Normally the battery 1 supplies current through the ignition switch 2 in its on position to both the radio 3 and the electricmotor-containing accessory 4.

The radio being considered is of the FM-AM variety. According to the state of the art, the AM portion 5 of the radio is conventional and employs permeability tuning, while the FM portion 6 employs electronic tuning with voltage variable capacitor elements providing the variable function of tuned circuits 7. A source of variable voltage 8 is used to provide the tuning voltage for the VVCs. The switching system uses a SPST switch 9 with an electronic circuit in place of the typical SPDT switch in the battery line. The circuit shown, which is claimed in a copending application Ser. No. 169,658 Filed Aug. 6, 1971, is preferred because the SPST switch is more reliable than a SPDT switch and, due to the transistor, the switch passes very little current thereby prolonging its life. The circuit operates as follows.

When the FM-AM switch 9 is in the FM position as shown, the PNP transistor 10 is biased to conduct and the FM and electronic tuning circuits 6 and 8 are energized. The germanium diode 11 is forward biased thereby lowering the voltage applied to the AM circuits to about 0.3 volt, a level too low for AM circuit operation. Resistor 12 provides the required voltage dropping action.

When switch 9 is open, or in the AM position, transistor 10 is biased nonconductive, thereby removing power from the FM circuits. Diode 11 is no longer conductive and'the AM circuits 5 are powered through resistor 12. Resistor 13 and capacitor 14 act as a power line filter. Resistor l5 limits the base current in transistor 10.

The following list shows the values of the parts of FIG. 1 that produce successful operation:

Transistor l0 silicon PNP 2N 5354 Diode ll germanium lN 6O Resistor 12 330 ohms Resistor l3 10 ohms Capacitor I4 500 microfarads Resistor 15 820 ohms With ignition switch 2 in the on position the radio will operate normally. When switch 16 of accessory 4 is on electric motor 17 is operating. Under these conditions, it can be seen that during rundown the electric motor 17 is connected to the radio and becomes the source of power when the ignition is switched off. During this condition the declining, or rundown, voltage derived from motor 17 is applied to the electronic tuning circuits thereby producing the tuning sweep and the undesirable noise.

In the prior art circuit, relay 18 would be omitted and points A and B connected together. To practice the invention, voltage sensitive relay 18 has its coil connected across the receiver power line, and the relay contacts are connected in series with the radio circuit. Relay 18 is adjusted to pull in at about 11 volts, the lowest expected battery voltage. Relay 18 is adjusted to drop out at about ,7 to 8 volts, the maximum line voltage value maintained by the coasting electric motor 17. When the ignition switch 2 is on relay 18 will pull in and energize the radio. When the ignition switch 2 is switched off relay 18 will drop out and remove power from the radio.

In FIG. 2 the circuit of FIG. 1 is shown, but relay 18 is omitted and diode 19 has been added to the transistor base circuit. Diode 19 is a 7.5 volt silicon zener diode such as type 1N958B. In terms of FM-AM switching the circuit operates as it did in FIG. 1. In the FM position where rundown is to be avoided, diode 19 will provide base current to the transistor under normal conditions to keep it conducting. When the ignition switch 2 is turned off, the emitter of the transistor will drop to about 7.5 volts under which condition zener diode 19 can no longer supply base current. With zener diode open,transistor 10 will no longer conduct and the FM circuits are turned off.

In general with a 7.5 volt zener diode, the transistor emitter must be in excess of about 8.2 volts for the FM circuits to operate. When the line voltage drops to the 7-8 volt level at the start of rundown, the circuit will act to terminate radio operation.

While the above description has been associated with automobile radio receivers, the invention can be applied to any electronically tuned receiver that has the declining voltage characteristic. In particular the circuits could be incorporated into the d-c supply of a solid state receiver having large filter capacitors. When the voltage drops below a predetermined value, the receiver circuits, at :least the electronic tuning circuits, are disconnected from the filter capacitors. Thus the rundown noise that would ordinarily be associated with shutting the receiver off is avoided. In addition, while two forms of switching have been shown, other equivalent circuits could be employed. The invention should be limited only by the following claims.

I claim:

1. In a radio receiver having voltage variable tuning elements, said receiver being subjected to conditions that result in declining supply voltage, said declining supply voltage producing undesirable noise in said receiver as said voltage variable tuning elements are swept during said decline, the improvement comprising: I

switching means connected in series with the voltage supply terminals of said receiver so as to terminate operation of said receiver upon actuation of said switching means and responsive to the decline of said supply voltage to a predetermined level for termination by actuation of said switch means the op eration of said receiver when said voltage falls to said level.

2. In the receiver of claim 1 wherein said switching means comprises an electrical relay adjusted to drop out at said predetermined level.

3. In the receiver of claim 1 wherein said switching means comprises an electronic switch having a zener diode to establish switching at said predetermined level.

4. In a radio receiver adapted to be connected to a source of voltage which source, when deenergized, continues to supply voltage to said radio receiver at a declining rate for a brief period after deenergization, said receiver employing voltage variable capacitors to effect tuning of said receiver in response to a voltage whose magnitude is a function of the voltage of said source, said voltage variable capacitors producing an undesirable noise in said receiver as they sweep the tuning of said receiver in response to said supply voltage decline, the improvement comprising switching means connected in series between said source and said radio receiver, and responsive to a decline in voltage of said source to a predetermined value, for terminating automatically the operation of said radio receiver.

5. In an automobile radio receiver having terminals adapted to be connected to the electrical system of an automobile, said automobile having, in addition to the conventional battery power supply system, accessories that act as sources of declining potential when disconnected from said battery power supply, said receiver employing a plurality of voltage variable capacitors to effect electronic tuning of said receiver in response to a voltage whose magnitude is a function of the voltage applied to said terminals, said voltage variable capacitors producing an undesirable noise in said receiver when subjected to said declining potential, the improvement comprising switching means connected in series between said terminals and said radio receiver circuits, and responsive to the application of substantially less than normal voltage to said terminals, for terminating automatically operation of said radio receiver. 

1. In a radio receiver having voltage variable tuning elements, said receiver being subjected to conditions that result in declining supply voltage, said declining supply voltage producing undesirable noise in said receiver as said voltage variable tuning elements are swept during said decline, the improvement comprising: switching means connected in series with the voltage supply terminals of said receiver so as to terminate operation of said receiver upon actuation of said switching means and responsive to the decline of said supply voltage to a predetermined level for termination by actuation of said switch means the operation of said receiver when said voltage falls to said level.
 2. In the receiver of claim 1 wherein said switching means comprises an electrical relay adjusted to drop out at said predetermined level.
 3. In the receiver of claim 1 wherein said switching means comprises an electronic switch having a zener diode to establish switching at said predetermined level.
 4. In a radio receiver adapted to be connected to a source of voltage which source, when deenergized, continues to supply voltage to said radio receiver At a declining rate for a brief period after deenergization, said receiver employing voltage variable capacitors to effect tuning of said receiver in response to a voltage whose magnitude is a function of the voltage of said source, said voltage variable capacitors producing an undesirable noise in said receiver as they sweep the tuning of said receiver in response to said supply voltage decline, the improvement comprising switching means connected in series between said source and said radio receiver, and responsive to a decline in voltage of said source to a predetermined value, for terminating automatically the operation of said radio receiver.
 5. In an automobile radio receiver having terminals adapted to be connected to the electrical system of an automobile, said automobile having, in addition to the conventional battery power supply system, accessories that act as sources of declining potential when disconnected from said battery power supply, said receiver employing a plurality of voltage variable capacitors to effect electronic tuning of said receiver in response to a voltage whose magnitude is a function of the voltage applied to said terminals, said voltage variable capacitors producing an undesirable noise in said receiver when subjected to said declining potential, the improvement comprising switching means connected in series between said terminals and said radio receiver circuits, and responsive to the application of substantially less than normal voltage to said terminals, for terminating automatically operation of said radio receiver. 